Uncalcined geopolymer-based refractory material and method for its preparation

ABSTRACT

Disclosed is a method for preparing an uncalcined geopolymer-based refractory material. The method includes the steps of mixing a mineral powder, a fly ash, a metakaolin, and silicon carbide whiskers by ball milling to form a milled material; mixing the milled material with a sodium water glass solution and water to form a slurry; and curing the slurry to obtain the uncalcined geopolymer-based refractory material. The uncalcined geopolymer-based refractory material thus prepared contains a geopolymer matrix formed of the mineral powder, the fly ash, and the metakaolin and the silicon carbide whiskers embedded in the geopolymer matrix.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a division of U.S. application Ser. No.17/474,908 filed on Sep. 14, 2021, which claims the benefit and priorityof Chinese Patent Application No. 202010959049.5, filed on Sep. 14,2020. The disclosures of both above-mentioned applications areincorporated by reference herein in their entirety as part of thepresent application.

TECHNICAL FIELD

The present disclosure is generally related to the field of buildingmaterials, and in particular to an uncalcined geopolymer-basedrefractory material and a method for its preparation.

BACKGROUND ART

Cement production process produces a large amount of carbon dioxide(CO₂). This goes against the requirement in modern society forsustainable development. There is thus a desire for new buildingmaterials. Geopolymers are inorganic polymers with a three-dimensionalnetwork structure mainly composed of tetrahedrally-coordinated AlO₄ andSiO₄ units, and have excellent mechanical properties and high resistanceto high temperatures and fire as well as acids and alkalis. Geopolymersare often synthesized via alkali activation technology, which is freefrom a so-called “two grinding and one calcination” process required bythe manufacturing of cement and thus has reduced energy consumption,resulting in lower CO₂ emission. Also, they can be made from variousindustrial waste materials (rich in active silicon and aluminum), whichavoids the problem that these waste materials would occupy the finiteland resources. However, geopolymers are brittle in nature, and suchdrawback becomes more conspicuous when they are exposed to a hightemperature environment. In that instance, there would be no chance forthe geopolymers to fully exhibit their excellent heat resistance.

SUMMARY

Accordingly, the present disclosure is directed to an uncalcinedgeopolymer-based refractory material exhibiting a ductile fracturemechanism instead of a brittle fracture mechanism and having excellentmechanical properties and heat resistance, and to a method for itspreparation.

In one aspect, the disclosure provides an uncalcined geopolymer-basedrefractory material, comprising a matrix of a geopolymer obtainable bypolymerization of a mixture consisting of mineral powder, fly ash, andmetakaolin; and silicon carbide (SiC) whiskers embedded in thegeopolymer matrix.

In an embodiment, the SiC whiskers are present in the geopolymer matrixin an amount of 0.8 to 1.2 wt. %.

In an embodiment, the SiC whiskers are composed of pure SiC only orboron nitride (BN) coated SiC. In a further embodiment, the BN coatedSiC whiskers have a 50 to 250 nm thick BN coating.

In an embodiment, the SiC whiskers have a diameter of 0.1 to 2.5 μm anda length of 2 to 50 μm.

In an embodiment, the mineral powder is high-calcium mineral powder, andthe fly ash is Class F fly ash. In an embodiment, a mass ratio ofmineral powder:fly ash:metakaolin is (35-45):(25-35):(25-35).

In another aspect, the present disclosure provides a method forpreparing the uncalcined geopolymer-based refractory material, themethod comprising:

-   -   (a) mixing the mineral powder, the fly ash, the metakaolin, and        the SiC whiskers by ball milling to form a milled material;    -   (b) mixing the milled material with a sodium water glass        solution (sodium silicate solution) and water to form a slurry;        and    -   (c) curing the slurry to obtain the uncalcined geopolymer-based        refractory material.

In some embodiments, the method further comprises: before step (a),subjecting the SiC whiskers to a dispersion treatment by mixing the SiCwhiskers with a dispersant solution by ultrasonic vibration, followed bydrying. In a particular embodiment, the dispersant solution is a 95 wt.% aqueous solution of 2-amino-2-methyl-1-propanol (AMP-95).

In an embodiment of the method of the disclosure, in step (a), themineral powder, the fly ash, the metakaolin, and the SiC whiskers areball milled together with zirconium oxide beads having a diameter of 5mm at a rotation speed of 150 rpm for 25 min.

In an embodiment of the method, the sodium water glass used in step (b)has an SiO₂/Na₂O modulus in the range from 1.5 to 3.5. In an embodiment,in step (b), a ratio of a mass of sodium silicate to a total mass of themineral powder, the fly ash, and the metakaolin is 0.15:1, and a ratioof a total mass of the water used and the water in the sodium waterglass solution to a total mass of the mineral powder, the fly ash, andthe metakaolin is 0.4:1.

In an embodiment, in step (c), the slurry obtained in step (b) is curedby placing it in a mold inside a curing chamber for 24 h at 95±5%relative humidity and 23±0.5° C. and then maintaining it inside thecuring chamber after removal from the mold for 7 days under the sametemperature and humidity conditions.

As described above, one aspect of the present disclosure provides anuncalcined geopolymer-based refractory material (also referred to hereinas a composite material or composite), comprising a matrix of ageopolymer obtainable by polymerization of a mixture consisting ofmineral powder, fly ash, and metakaolin; and silicon carbide (SiC)whiskers embedded in the geopolymer matrix. It has been found that theincorporation of the SiC whiskers can increase the fracture toughness ofthe material by exhausting more fracture energy via mechanisms includingwhisker debonding, whisker pull-out and crack deflection. The presentcomposite material not only exhibits all of the advantages of thegeopolymer matrix material, but is also toughened by the SiC whiskerssuch that the material is hardly brittle-fractured even at hightemperatures (at which pure geopolymer materials that are not toughenedby SiC whiskers tend to undergo brittle fracture due to excessively highvapour pressure inside pores of these materials exceeding the maximumpressure limit that can be withstood by the pore walls) since the SiCwhiskers can exhibit good performances at high temperatures.Accordingly, the present composite material can have an improvedresistance to high temperatures and fire. The present material can haveexcellent mechanical properties and resistance to high temperatures andexhibit a ductile fracture mechanism instead of a brittle fracturemechanism. In addition, the geopolymer component of the composite isobtainable from mineral powder, fly ash, and metakaolin, each of whichis low in cost.

Another aspect of the present disclosure provides a method for preparingthe uncalcined geopolymer-based refractory material by usingalkali-activation technology. The method is free from the “two grindingand one calcination” as mentioned in the background, and thus hasreduced energy consumption. Moreover, this method is simple and easy toimplement, and can be applied on an industrial scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the load displacement curves of a four point bendtest of the materials prepared in Examples 1 and 2 and in ComparativeExample.

DETAILED DESCRIPTION

In one aspect, the present disclosure provides an uncalcinedgeopolymer-based refractory material or a composite material, comprisinga matrix of a geopolymer obtainable by polymerization of a mixtureconsisting of mineral powder, fly ash, and metakaolin; and siliconcarbide (SiC) whiskers embedded in the geopolymer matrix.

The composite material comprises a geopolymer matrix. The geopolymer isobtainable by polymerization of a mixture consisting of mineral powder,fly ash, and metakaolin. In an embodiment, the mineral power ishigh-calcium mineral powder having a calcium content, present as calciumoxide, of greater than or equal to 40 wt. %. In an embodiment, the flyash is Class F fly ash. In an embodiment, a mass ratio of mineralpowder:fly ash:metakaolin is (35-45):(25-35):(25-35), preferably 4:3:3.

The composite material of the present disclosure further comprises SiCwhiskers embedded in the geopolymer matrix.

In an embodiment, the SiC whiskers are present in the geopolymer matrixin an amount of 0.8 to 1.2 wt. %, preferably 1 wt. %. In a particularembodiment, the SiC whiskers are composed of pure SiC only or boronnitride (BN) coated SiC. In a further embodiment, the BN coated SiCwhiskers have a 50 to 250 nm thick, preferably 150 to 200 nm thick, BNcoating. In an embodiment, the SiC whiskers have a diameter of 0.1 to2.5 μm, preferably 0.5 to 2 μm, and a length of 2 to 50 μm, preferably10 to 40 μm.

In some embodiments, the BN coated SiC whiskers can be obtained by aprocess comprising:

-   -   (i) mixing and heating boric acid, urea, acrylamide (AM),        N,N′-methylene bisacrylamide (MBA), water, ethanol, and ammonium        persulphate to form a solution;    -   (ii) immersing pure SiC whiskers in the solution, followed by        drying and sintering.

According to these embodiments, in order to obtain the BN coated SiCwhiskers, boric acid, urea, AM, MBA, water, ethanol, and ammoniumpersulphate are first mixed together in proportion and heated to form asolution. A preferred mass ratio of the boric acid to the urea is 1:1.5.A preferred mass ratio of AM:MBA:boric acid is 25:1:100. A preferredmass ratio of the water to the boric acid is 2.5:1. A preferred massratio of the ammonium persulphate to the boric acid is 3:100. Theethanol is preferably anhydrous ethanol. A preferred mass ratio of theethanol to the boric acid is 5:1 (unit: mL/g). The ethanol is added toaid solubility.

The step of mixing boric acid, urea, AM, MBA, water, ethanol, andammonium persulphate may be performed by adding boric acid, urea, AM,and MBA to water and mixing to form a first solution; adding ethanol tothe first solution and mixing to form a second solution; adding ammoniumpersulphate to the second solution and mixing to form the solution instep (i). The manner of mixing is not particularly limited and may beaccomplished in any conventional fashion, such as, for example,stirring.

The heating in step (i) is preferably carried out at 60° C. The heatingmay be carried out for any time period sufficient to ensure completedissolution. The heating temperature is preferably kept constant by athermostatically regulated water bath.

Pure SiC whiskers are then immersed in the solution in step (i) and thendried and sintered in order to obtain the BN coated SiC whiskers. Theimmersion procedure in the solution may be repeated twice, 10 min each.The drying temperature and time period are not particularly limited aslong as the immersed SiC whiskers can be dried to constant weight.

The sintering may be carried out at 850° C. in a tube furnace for 2 h.During the sintering process, the boric acid and the urea are reactedwith each other in the presence of cross-linking agents, i.e., AM andMBA, and of an initiator, i e , ammonium persulphate, to form BN, withwhich the pure SiC whiskers are coated. The BN coating enables animprovement in the resistance of the whiskers to oxidation, reducingsurface damage that can occur during oxidation. Furthermore, the BNcoating can compensate for the thermal expansion coefficient mismatchbetween the SiC whiskers and the geopolymer matrix, and reduce thetangential stress at interface surfaces at high temperatures, resultingin enhanced debonding and pull-out behaviors of the whiskers in thecomposite material and thus better ductility and fracture resistance ofthe material.

It has also been found that the incorporation of SiC whiskers into thegeopolymer matrix can increase the fracture toughness of the compositeby exhausting more fracture energy via whisker debonding, whiskerpull-out and crack deflection. The present composite material not onlyexhibits all of the advantages of the geopolymer matrix material, but isalso toughened by the SiC whiskers such that the material is hardlybrittle-fractured even at high temperatures (at which pure geopolymermaterials that are not toughened by SiC whiskers tend to undergo brittlefracture) since the SiC whiskers can exhibit good performances at hightemperatures. Accordingly, the composite material can have an improvedresistance to high temperatures over the geopolymer. The presentmaterial can have excellent mechanical properties and high resistance tohigh temperatures and exhibit a ductile fracture mechanism instead of abrittle fracture mechanism. In addition, the geopolymer component of thecomposite is obtainable from mineral powder, fly ash, and metakaolin,each of which is low in cost.

In another aspect, the present disclosure provides a method forpreparing the uncalcined geopolymer-based refractory material, themethod comprising:

-   -   (a) mixing the mineral powder, the fly ash, the metakaolin, and        the SiC whiskers by ball milling to form a milled material;    -   (b) mixing the milled material with a sodium water glass        solution (sodium silicate solution) and water to form a slurry;        and    -   (c) curing the slurry to obtain the uncalcined geopolymer-based        refractory material.

The mineral powder, the fly ash, the metakaolin, and the SiC whiskersare first mixed by ball milling to form a milled material. In anembodiment, the mineral powder is high-calcium mineral powder. In anembodiment, the fly ash is Class F fly ash. In an embodiment, themineral powder, the fly ash, and the metakaolin are mixed in a mixingratio (mass ratio) of (35-45):(25-35):(25-35), preferably 4:3:3. Thesource of any of the mineral powder, the fly ash, the metakaolin is notparticularly limited. Preferably, however, the mineral powder, the flyash, and the metakaolin are derived from waste industrial materials.This advantageously may achieve reuse of these waste materials and avoidthe problem that they would occupy the finite land resources. In anembodiment, the SiC whiskers are used in an amount of 0.8 to 1.2 wt. %,preferably 1 wt. %, with respect to the total use amount of the mineralpowder, the fly ash, and the metakaolin.

In some embodiments, the method further comprises: before step (a),subjecting the SiC whiskers to a dispersion treatment by mixing the SiCwhiskers with a dispersant solution by ultrasonic vibration, followed bydrying of the mixture. In a particular embodiment, the dispersantsolution is a 95 wt. % aqueous solution of 2-amino-2-methyl-1-propanol(AMP-95).

A mass ratio of the dispersant solution to the SiC whiskers may be 1:10.The ultrasonic vibration may be carried out in any suitable manner, andthe ultrasonic vibration conditions are not particular limited as longas the tangling between the whiskers can be broken. The temperature andperiod of time for drying the mixture of the SiC whiskers and thedispersant solution are not particularly limited as long as they can bedried to constant weight. During the drying of the mixture, the waterand AMP-95 contents in the mixture are evaporated, leaving the dispersedSiC whiskers.

In an embodiment, in step (a), the mineral powder, the fly ash, themetakaolin, and the SiC whiskers are ball milled together with zirconiumoxide beads having a diameter of 5 mm at a rotation speed of 150 rpm for25 min, to form a milled material. Ball milling can ensure that themixture is uniformly mixed and dispersed.

After the milled material is formed, it is mixed with a sodium waterglass solution and water to form a slurry. In an embodiment, the sodiumwater glass has an SiO₂/Na₂O modulus in the range from 1.5 to 3.5,preferably 2. In an embodiment, the sodium water glass solution has awater content of 60 wt. %. In an embodiment, a ratio of a mass of sodiumsilicate in the sodium water glass solution to a total mass of themineral powder, the fly ash, and the metakaolin is 0.15:1. In anembodiment, a ratio of a total mass of the water used and the water inthe sodium water glass solution to a total mass of the mineral powder,the fly ash, and the metakaolin is 0.4:1. The water used in this stepmay be deionized water. The milled material may be mixed with the sodiumwater glass solution and the water by stirring at high speed, preferablyat 100 to 130 rpm. The stirring may be carried out for any timesufficient to ensure uniform mixing.

After the slurry is obtained, it is cured so as to obtain the uncalcinedgeopolymer-based refractory material. In an embodiment, the slurry iscured by placing it in a mold inside a curing chamber for 24 h at 95±5%relative humidity and 23±0.5° C. and then maintaining it inside thecuring chamber after removal from the mold for 7 days under the sametemperature and humidity conditions.

Therefore, as described above, the present disclosure also provides amethod for preparing the uncalcined geopolymer-based refractory materialby using alkali-activation technology. The method is free from the “twogrinding and one calcination” as mentioned in the background, and thushas reduced energy consumption. Moreover, this method is simple and easyto implement, and can be applied on an industrial scale.

The disclosure will now be further illustrated by the following exampleswhich are not intended to limit the scope of the disclosure in any wall.

Example 1

5 g of pure SiC whiskers with a diameter of 0.1 to 2.5 μm and a lengthof 2 to 50 μm were added to a 95 wt. % aqueous solution of AMP-95 anddispersed by using an ultrasonic washing machine, followed by drying.

5 g of the above obtained SiC whiskers, 200 g of high-calcium mineralpowder, 150 g of Class F fly ash, and 150 g of metakaolin were mixed anddispersed by a ball mill using zirconium oxide beads having a diameterof 5 mm at a rotation speed of 150 rpm for 25 min, to give a milledmaterial.

To a concrete mixer, the milled material, 187.5 g of a sodium waterglass solution (2 modulus; 60 wt. % water content), and 87.5 g ofdeionized water were added and stirred at a high speed of 120 rpm for 3min to form a slurry.

The slurry was poured into a mold and then placed into a concrete curingbox for curing at 95±5% relative humidity and 23±0.5° C. for 24 h andthen for 7 days in the curing box at the same conditions after removalfrom the mold. Finally, a SiC whiskers reinforced geopolymer material(i.e., an uncalcined geopolymer-based refractory material) was obtained.

Example 2

4 g of boric acid, 6 g of urea, 1 g of AM, and 0.04 g of MBA were addedto 10 mL of deionized water in a beaker. 20 mL of anhydrous ethanol andthen 0.12 g of ammonium persulphate were added thereto. Then, the beakerwas placed in a water bath (60° C.) and stirring was performed until allsolids were dissolved. A amount of pure SiC whiskers (having a diameterof 0.1 to 2.5 μm and a length of 2 to 50 μm) were immersed in thesolution. The immersion operation was carried out twice for 10 min each.Then, filtration and drying were performed. Thereafter, the resultingwhiskers were kept at 850° C. in a tube furnace for 2 h so that the SiCwhiskers were coated with a 150 nm thick BN coating.

5 g of the BN coated SiC whiskers were weighed, added to 0.5 g of a 95wt. % aqueous solution of AMP-95 and dispersed by using an ultrasonicwashing machine, followed by drying.

5 g of the above obtained BN coated SiC whiskers, 200 g of high-calciummineral powder, 150 g of Class F fly ash, and 150 g of metakaolin weremixed and dispersed by a ball mill using zirconium oxide beads having adiameter of 5 mm at a rotation speed of 150 rpm for 25 min, to give amilled material.

To a concrete mixer, the milled material, 187.5 g of a sodium waterglass solution (2 modulus; 60 wt. % water content), and 87.5 g ofdeionized water were added and stirred at a high speed of 120 rpm for 3min to form a slurry.

The slurry was poured into a mold and then placed into a concrete curingbox for curing at 95±5% relative humidity and 23±0.5° C. for 24 h andthen for 7 days in the curing box at the same conditions after removalfrom the mold. Finally, a BN coated SiC whiskers reinforced geopolymermaterial (i.e., uncalcined geopolymer-based refractory material) wasobtained.

Comparative Example

200 g of high-calcium mineral powder, 150 g of Class F fly ash, and 150g of metakaolin were mixed and dispersed by a ball mill using zirconiumoxide beads having a diameter of 5 mm at a rotation speed of 150 rpm for25 min, to give a milled material.

To a concrete mixer, the milled material, 187.5 g of a sodium waterglass solution (2 modulus; 60 wt. % water content), and 87.5 g ofdeionized water were added and stirred at a high speed of 120 rpm for 3min to form a slurry.

The slurry was poured into a mold and then placed into a concrete curingbox for curing at 95±5% relative humidity and 23±0.5° C. for 24 h andthen for 7 days in the curing box at the same conditions after removalfrom the mold. Finally, a pure geopolymer material was obtained.

The materials prepared in Examples 1 and 2 and in Comparative Examplewere each made into a rectangular bar with dimensions of 5 mm×5 mm×29 mmand tested for their bending strength at different temperatures byperforming a four point bending test using a span length of 17 mm. Thetest results are shown in Table 1 below.

TABLE 1 Bending Strength/ MPa Temperature Comp Ex. Ex. 1 Ex. 2 R.T. 7.498.14 9.76 600 ° C. 8.05 10.74 11.45 900 ° C. 0 (indicating severe 6.919.05 cracking)

From Table 1 it is evident that the composite materials prepared inExamples 1 and 2 exhibited better mechanical properties and resistanceto high temperatures than the pure geopolymer material in ComparativeExample.

FIG. 1 is a graph of the load displacement curves of the four point bendtest of the materials prepared in Examples 1 and 2 and in ComparativeExample, where A, B, and C represent the curve of Examples 1 and 2 andComparative Example, respectively. From this figure, it can be foundthat the pure geopolymer material prepared in Comparative Exampleexhibited a typical brittle failure, while the SiC whiskers reinforcedgeopolymer material prepared in Example 1 exhibited a ductile failure,and thus had an improved fracture toughness over the pure geopolymermaterial. Further, the BN coated SiC whiskers reinforced geopolymermaterial prepared in Example 2 exhibited a further improved mechanicalstrength and fracture toughness over the material in Example 1.

In conclusion, the materials of the disclosure can exhibit a ductilefailure instead of a brittle failure, and have excellent mechanicalproperties and high resistance to high temperatures.

The descriptions above are just preferred embodiments of the disclosure.Accordingly, those skilled in the art will recognize that variouschanges and modifications of the embodiments described herein can bemade without departing from the scope and spirit of the disclosure.

What is claimed is:
 1. A method for preparing an uncalcinedgeopolymer-based refractory material, the method comprising the stepsof: (a) mixing a mineral powder, a fly ash, a metakaolin, and siliconcarbide whiskers by ball milling to form a milled material; (b) mixingthe milled material with a sodium water glass solution and water to forma slurry; and (c) curing the slurry to obtain the uncalcinedgeopolymer-based refractory material, wherein the uncalcinedgeopolymer-based refractory material comprises (i) a geopolymer matrixformed by polymerization of a mixture consisting of the mineral powder,the fly ash, and the metakaolin, and (ii) the silicon carbide whiskersembedded in the geopolymer matrix.
 2. The method of claim 1, furthercomprising, before step (a), subjecting the silicon carbide whiskers toa dispersion treatment by mixing the silicon carbide whiskers with adispersant solution by ultrasonic vibration, followed by drying, whereinthe dispersant solution is a 95 wt. % aqueous solution of2-amino-2-methyl-1-propanol.
 3. The method of claim 1, wherein, in step(a), the mineral powder, the fly ash, the metakaolin, and the siliconcarbide whiskers are ball milled together with zirconium oxide beadshaving a diameter of 5 mm at a rotation speed of 150 rpm for 25 min. 4.The method of claim 1, wherein the sodium water glass used in step (b)has a SiO₂/Na₂O modulus in the range from 1.5 to 3.5, the ratio of themass of sodium silicate in the sodium water glass solution to the totalmass of the mineral powder, the fly ash, and the metakaolin is 0.15:1,and the ratio of the total mass of the water used in step (b) and thewater in the sodium water glass solution to the total mass of themineral powder, the fly ash, and the metakaolin is 0.4:1.
 5. The methodof claim 1, wherein, in step (c), the slurry obtained in step (b) iscured by placing it in a mold inside a curing chamber for 24 h at 95±5%relative humidity and 23±0.5° C. and then maintaining it inside thecuring chamber after removal from the mold for 7 days under the sametemperature and humidity conditions.
 6. The method of claim 1, whereinthe silicon carbide whiskers are present in the geopolymer matrix in anamount of 0.8 to 1.2 wt.%.
 7. The method of claim 1, wherein the siliconcarbide whiskers, consisting of pure silicon carbide or boron nitridecoated silicon carbide, have a diameter of 0.1 to 2.5 μm and a length of2 to 50 μm.
 8. The method of claim 6, wherein the silicon carbidewhiskers, consisting of pure silicon carbide only or boron nitridecoated silicon carbide, have a diameter of 0.1 to 2.5 μm and a length of2 to 50 μm.
 9. The method of claim 7, wherein the boron nitride coatedsilicon carbide whiskers have a 50 to 250 nm thick boron nitridecoating.
 10. The method of claim 8, wherein the boron nitride coatedsilicon carbide whiskers have a 50 to 250 nm thick boron nitridecoating.
 11. The method of claim 1, wherein the mineral powder is ahigh-calcium mineral powder, the fly ash is a Class F fly ash, and themass ratio of mineral powder:fly ash:metakaolin is(35-45):(25-35):(25-35).
 12. The method of claim 6, further comprising,before step (a), subjecting the silicon carbide whiskers to a dispersiontreatment by mixing the silicon carbide whiskers with a dispersantsolution by ultrasonic vibration, followed by drying, wherein thedispersant solution is a 95 wt % aqueous solution of2-amino-2-methyl-1-propanol.
 13. The method of claim 7, furthercomprising, before step (a), subjecting the silicon carbide whiskers toa dispersion treatment by mixing the silicon carbide whiskers with adispersant solution by ultrasonic vibration, followed by drying, whereinthe dispersant solution is a 95 wt % aqueous solution of2-amino-2-methyl-1-propanol.